Elastic Wave Scattering From a Partially Debonded Elastic Cylindrical Inclusion

The problem of elastic wave scattering from a partially debonded elastic cylindrical inclusion is investigated by using the wave function expansion method and singular integral equation technique. The debonding regions are modeled as interface cracks with non-contacting faces. The mixed boundary conditions of the problem lead to a set of simultanious dual series equations which, by introducing the dislocation density functions as unknowns, can be further converted to a set of singular integral equations of the second type. Solving these equations numerically, we obtain the dynamic stress intensity factor(DSIF), the scattered far-field pattern and the scattering cross section(SCS). The numerical results for an inclusion with one debond are presented to show the distinguishing feature of the problem-the low frequency resonances in both DSIF and SCS. Finally, as a special example, we discuss the scattering of elastic waves by a circular arc-shaped crack in a homogeneous medium. The presented method is valid when stresses have oscillatory behavior.

DYNAMIC ANALYSIS OF A BURIED RIGID ELLIPTIC CYLINDER PARTIALLY DEBONDED FROM SURROUNDING MATRIX UNDER SHEAR WAVES

This paper investigates the dynamic behavior of a buried rigid elliptic cylinder partially debonded from surrounding matrix under the action of anti-plane shear waves (SH waves). The debonding region is modeled as an elliptic arc-shaped interface crack with non-contacting faces. By using the wave function (Mathieu function) expansion method and introducing the dislocation density function as an unknown variable, the problem is reduced to a singular integral equation which is solved numerically to calculate the near and far fields of the problem. The resonance of the structure and the effects of various parameters on the resonance are discussed.

ANALYSIS OF A PARTIALLY DEBONDED ELLIPTIC INHOMOGENEITY IN PIEZOELECTRIC MATERIALS

A generalized solution was obtained for the partially debonded elliptic inhomogeneity problem in piezoelectric materials under antiplane shear and inplane electric loading using the complex variable method. It was assumed that the interfacial debonding induced an electrically impermeable crack at the interface. The principle of conformal transformation and analytical continuation were employed to reduce the formulation into two Riemann-Hilbert problems. This enabled the determination of the complex potentials in the inhomogeneity and the matrix by means of series of expressions. The resulting solution was then used to obtain the electroelastic fields and the energy release rate involving the debonding at the inhomogeneity-matrix interface. The validity and versatility of the current general solution have been demonstrated through some specific examples such as the problems of perfectly bonded elliptic inhomogeneity, totally debonded elliptic inhomogeneity, partially debonded rigid and conducting elliptic inhomogeneity, and partially debonded circular inhomogeneity.

Bending strength degradation of a cantilever plate with surface energy due to partial debonding at the clamped boundary

This paper investigates the bending fracture problem of a micro/nanoscale cantilever thin plate with surface energy,where the clamped boundary is partially debonded along the thickness direction.Some fundamental mechanical equations for the bending problem of micro/nanoscale plates are given by the Kirchhoff theory of thin plates,incorporating the Gurtin-Murdoch surface elasticity theory.For two typical cases of constant bending moment and uniform shear force in the debonded segment,the associated problems are reduced to two mixed boundary value problems.By solving the resulting mixed boundary value problems using the Fourier integral transform,a new type of singular integral equation with two Cauchy kernels is obtained for each case,and the exact solutions in terms of the fundamental functions are determined using the PoincareBertrand formula.Asymptotic elastic fields near the debonded tips including the bending moment,effective shear force,and bulk stress components exhibit the oscillatory singularity.The dependence relations among the singular fields,the material constants,and the plate's thickness are analyzed for partially debonded cantilever micro-plates.If surface energy is neglected,these results reduce the bending fracture of a macroscale partially debonded cantilever plate,which has not been previously reported.

A Comparative Study on the Post-Buckling Behavior of Reinforced Thermoplastic Pipes(RTPs)Under External Pressure Considering Progressive Failure

The collapse pressure is a key parameter when RTPs are applied in harsh deep-water environments.To investigate the collapse of RTPs,numerical simulations and hydrostatic pressure tests are conducted.For the numerical simulations,the eigenvalue analysis and Riks analysis are combined,in which the Hashin failure criterion and fracture energy stiffness degradation model are used to simulate the progressive failure of composites,and the“infinite”boundary conditions are applied to eliminate the boundary effects.As for the hydrostatic pressure tests,RTP specimens were placed in a hydrostatic chamber after filled with water.It has been observed that the cross-section of the middle part collapses when it reaches the maximum pressure.The collapse pressure obtained from the numerical simulations agrees well with that in the experiment.Meanwhile,the applicability of NASA SP-8007 formula on the collapse pressure prediction was also discussed.It has a relatively greater difference because of the ignorance of the progressive failure of composites.For the parametric study,it is found that RTPs have much higher first-ply-failure pressure when the winding angles are between 50°and 70°.Besides,the effect of debonding and initial ovality,and the contribution of the liner and coating are also discussed.

Effect of neutral polymeric bonding agent on tensile mechanical properties and damage evolution of NEPE propellant

Introducing Neutral Polymeric bonding agents(NPBA) into the Nitrate Ester Plasticized Polyether(NEPE)propellant could improve the adhesion between filler/matrix interface, thereby contributing to the development of new generations of the NEPE propellant with better mechanical properties. Therefore,understanding the effects of NPBA on the deformation and damage evolution of the NEPE propellant is fundamental to material design and applications. This paper studies the uniaxial tensile and stress relaxation responses of the NEPE propellant with different amounts of NPBA. The damage evolution in terms of interface debonding is further investigated using a cohesive-zone model(CZM). Experimental results show that the initial modulus and strength of the NEPE propellant increase with the increasing amount of NPBA while the elongation decreases. Meanwhile, the relaxation rate slows down and a higher long-term equilibrium modulus is reached. Experimental and numerical analyses indicate that interface debonding and crack propagation along filler-matrix interface are the dominant damage mechanism for the samples with a low amount of NPBA, while damage localization and crack advancement through the matrix are predominant for the ones with a high amount of NPBA. Finally, crosslinking density tests and simulation results also show that the effect of the bonding agent is interfacial rather than due to the overall crosslinking density change of the binder.

Parametric study on contact sensors for MASW measurement-based interfacial debonding detection for SCCS

Steel-concrete composite structures(SCCS)have been widely used as primary load-bearing components in large-scale civil infrastructures.As the basis of the co-working ability of steel plate and concrete,the bonding status plays an essential role in guaranteeing the structural performance of SCCS.Accordingly,efficient non-destructive testing(NDT)on interfacial debondings in SCCS has become a prominent research area.Multi-channel analysis of surface waves(MASW)has been validated as an effective NDT technique for interfacial debonding detection for SCCS.However,the feasibility of MASW must be validated using experimental measurements.This study establishes a high-frequency data synchronous acquisition system with 32 channels to perform comparative verification experiments in depth.First,the current sensing approaches for high-frequency vibration and stress waves are summarized.Secondly,three types of contact sensors,namely,piezoelectric lead-zirconate-titanate(PZT)patches,accelerometers,and ultrasonic transducers,are selected for MASW measurement.Then,the selection and optimization of the force hammer head are performed.Comparative experiments are carried out for the optimal selection of ultrasonic transducers,PZT patches,and accelerometers for MASW measurement.In addition,the influence of different pasting methods on the output signal of the sensor array is discussed.Experimental results indicate that optimized PZT patches,acceleration sensors,and ultrasonic transducers can provide efficient data acquisition for MASW-based non-destructive experiments.The research findings in this study lay a solid foundation for analyzing the recognition accuracy of contact MASW measurement using different sensor arrays.

Determination of interfacial adhesive properties for polymeric film by blister test

The interfacial adhesive properties ofpolypropylene/stainless steel were studied by the blister test. The polypropylene film with a squared free-standing window was pressured by oil from one side of film. The corresponding deformation field was observed by a digital speckle correlation method. The experimental results show that the squared film deforms and debonds from stainless steel with the increase of pressure. The debonding of the squared film in initiates from the center of edge and extends to the comer, and then the deformation of film evolves from square to circle shape. The interfacial adhesive energy of polypropylene/stainless steel is （22.60±1.55） J/m2, which is in agreement with that measured by film with a circular window.

EFFECT OF FIBER FAILURE ON QUASI-STATIC UNLOADING/RELOADING HYSTERESIS LOOPS OF CERAMIC MATRIX COMPOSITES

The two-parameter Weibull model is used to describe the fiber strength distribution.The stress carried by the intact and fracture fibers on the matrix crack plane during unloading/reloading is determined based on the global load sharing criterion.The axial stress distribution of intact fibers upon unloading and reloading is determined based on the mechanisms of fiber sliding relative to matrix in the interface debonded region.The interface debonded length,unloading interface counter slip length,and reloading interface new slip length are obtained by the fracture mechanics approach.The hysteresis loops corresponding to different stresses considering fiber failure are compared with the cases without considering fiber failure.The effects of fiber characteristic strength and fiber Weibull modulus on the fiber failure,the shape,and the area of the hysteresis loops are analyzed.The predicted quasi-static unloading/reloading hysteresis loops agree well with experimental data.

Debonding phenomenon of TiO_2 nanotube film

Curvature method was used to measure the residual stress and substrate straining tensile test was carried out to study the debonding behavior of TiO2 nanotube film. The results indicate that the internal residual stress is -54 MPa. The strains of debonding initiation of TiO2 nanotube films without annealing, with 250 °C annealing and with 400 °C annealing are 2.6%, 5.1% and 8.6%, respectively, and the average radii of the debonding patches with debonding initiation are 27.5, 17.1 and 19.4 μm, respectively. The true critical debonding stresses of TiO2 nanotube films without annealing, with 250 °C annealing and with 400 °C annealing can be estimated as 220.4, 394.5 and 627.9 MPa, respectively. Interfacial shear lag model is modified and polynomial fitting equation of the interfacial shear strength of TiO2 nanotube film is demonstrated under debonding conditions. The modification and polynomial fitting are reliable since good agreement of the interfacial shear strengths after fitting is obtained compared with those results from the crack density analysis.

Compressive response and microstructural evolution of in-situ TiB_(2)particle-reinforced 7075 aluminum matrix composite

The hot forming behavior,failure mechanism,and microstructure evolution of in-situ TiB_(2)particle-reinforced 7075 aluminum matrix composite were investigated by isothermal compression test under different deformation conditions of deformation temperatures of 300−450℃ and strain rates of 0.001^(−1)s^(−1).The results demonstrate that the failure behavior of the composite exhibits both particle fracture and interface debonding at low temperature and high strain rate,and dimple rupture of the matrix at high temperature and low strain rate.Full dynamic recrystallization,which improves the composite formability,occurs under conditions of high temperature(450℃)and low strain rate(0.001 s^(−1));the grain size of the matrix after hot compression was significantly smaller than that of traditional 7075Al and ex-situ particle reinforced 7075Al matrix composite.Based on the flow stress curves,a constitutive model describing the relationship of the flow stress,true strain,strain rate and temperature was proposed.Furthermore,the processing maps based on both the dynamic material modeling(DMM)and modified DMM(MDMM)were established to analyze flow instability domain of the composite and optimize hot forming processing parameters.The optimum processing domain was determined at temperatures of 425−450℃ and strain rates of 0.001−0.01 s^(−1),in which the fine grain microstructure can be gained and particle crack and interface debonding can be avoided.

INTERFACIAL DEBONDING OF COATED-FIBER-REINFORCED COMPOSITES UNDER TENSION-TENSION CYCLIC LOADING

A new degradation function of the friction coefficient is used.Based on the double shear-lag model and Paris formula,the interracial damage of coated- fiber-reinforced composites under tension-tension cyclic loading is studied.The effects of strength and thickness of the coating materials on the debond stress,debond rate as well as debond length are simulated.

Numerical Simulation of Particle/Matrix Interface Failure in Composite Propellant

Interface debonding between particle and matrix in composite propellant influences its macroscopic mechanical properties greatly. For this, the laws of interface cohesive damage and failure were analyzed. Then, its microscopic computational model was established. The interface mechanical response was modeled by the bilinear cohesive zone model. The effects of interface properties and particle sizes on the macroscopic mechanical behavior were investigated. Numerical simulation of debonding damage evolution of composite propellant under finite deformation was carried out. The debonding damage nucleation, propagation mechanism and non-uniform distribution of microscopic stress-strain fields were discussed. The results show that the finite element simulation method based on microstructure model can effectively predict the trend of macroscopic mechanical behavior and particle/matrix debonding evolution process. It can be used for damage simulation and failure assessment of composite propellants.

Study on ASTM Shear-loaded Adhesive Lap Joints

Finite element analyses and experiments are conducted to analyze the mechanical behavior of ASTM shear-loaded adhesive lap joints. Adhesive is characterized for the stress-strain relation by comparing the apparent shear-strain relations obtained from finite element analysis and experiments following ASTM D 5656 Standard. With the established stress-strain relation, two failure criteria using equivalent plastic strain and J-integral are adopted to predict the failure loads for joint specimens following ASTM D 5656 and ASTM D 3165 Standard, respectively. Good correlation is found between the finite element results and the experimental results. The strength of ASTM D 3165 specimens with debonding defects is also studied. Calculation results shows that experiment data following the standards provide only relative material constants, such as apparent shear modulus and strengths. Further investigation is required to find out the engineering properties needed for actual joint design. For the specimens with debonding defects, the locations of defects have great effects on their load bearing ability.

Flexural Performance of CFRP-Bamboo Scrimber Composite Beams

This study presents a new structure made up of bamboo scrimber and carbon fiber reinforced polymer(CFRP)to address the low stiffness and strength of bamboo scrimbers.Three-point bending test and finite element model were conducted to study the failure mode,strain-displacement relationship,load-displacement relationship and relationships between strain distribution,contact pressure and deflection,and adhesive debonding.The results indicated that the flexural modulus and static flexural strength of the composite beams were effectively increased thanks to the CFRP sheets.The flexural modulus of the composite specimens were 2.33-2.94 times that of bamboo scrimber beams,and the flexural strength were 1.49-1.58 times that of bamboo scrimber beams.Adhesive debonding had a great influence on the strain distribution and deflection of the composite specimens.It was an important factor for the failure of the CFRP-bamboo scrimber composite specimens.According to the finite element simulation,the strain distribution,contact pressure and deflection also greatly changed with the adhesive debonding.After complete peeling,the deflection of the specimen was 3.09 times that of the unpeeled because it was no longer an integral beam.

Sandstone-concrete interface transition zone (ITZ) damage and debonding micromechanisms under freeze-thaw

The sufficient bond between concrete and rock is an important prerequisite to ensure the effect of shotcrete support. However, in cold regions engineering protection system, the bond condition of rock and concrete surface is easily affected by freeze-thaw cycles, resulting in interface damage, debonding and even supporting failure. Understanding the micromechanisms of the damage and debonding of the rock-concrete interface is essential for improving the interface protection.Therefore, the micromorphology, micromechanical properties, and microdebonding evolution of the sandstone-concrete interface transition zone(ITZ) under varying freeze-thaw cycles(0, 5, 10, 15, 20) were studied using scanning electron microscope, stereoscopic microscope, and nano-indentation. Furthermore, the distribution range and evolution process of ITZ affected by freeze-thaw cycles were defined. Major findings of this study are as follows:(1) The microdamage evolution law of the ITZ under increasing freeze-thaw cycles is clarified, and the relationship between the number of cracks in the ITZ and freeze-thaw cycles is established;(2) As the number of freeze-thaw cycles increases, the ITZ's micromechanical strength decreases, and its development width tends to increase;(3) The damage and debonding evolution mechanisms of sandstone-concrete ITZ under freeze-thaw cycles is revealed, and its micromechanical evolution model induced by freeze-thaw cycles is proposed.

MECHANICAL PERFORMANCE OF NANO-CaCO_3 FILLED POLYSTYRENE COMPOSITES

Nano-CaCO3 incorporated polystyrene composites are compounded by twin-screw extrusion. Tensile and compact tensile tests show that the strength and toughness of polystyrene are decreased after the addition of nano-CaCO3 particles. Fracture surface analysis suggests that the defects induced by interfacial debonding and nano-filler agglomerations would be the key factors responsible for the declined strength and toughness. Nevertheless, it has to be stated, if the applied stress is lower than the ultimate strength, the rigid nanoparticles would still stiffen the polymer molecules, and resist polymer chain mobility. Hence, the improved tensile modulus and creep resistance can be obtained with the increasing contents of nanoparticles.

Debonding Failure in FRP Reinforced SHCC Beams Induced from MultipleFlexural-Shear Cracks under Three-Point Bending Test

Strain hardening cement-based composites(SHCC)beam externally bonded with glass fiber-reinforced polymer(FRP)plate was examined under three-point flexural test.The effects of the type of substrate used(plain cement mortar vs.SHCC),the use or not of a FRP plate to strengthen the SHCC beam,and the thickness of the FRP plate on the flexural performances were studied.Results show that the ultimate load of SHCC beams strengthened with FRP plate has improved greatly in comparison with plain SHCC beams.The deformation capacity of beams makes little change with an increase in the thickness of FRP plates.The formation of multiple flexural-shear cracks(MFSC)is the unique feature of SHCC beams bonded with FRP plates under three-point bending.The debonding of the FRP plate is initiated from MFSC.The initiated debonding area(IDA)is formed by the joint points of the flexural-shear cracks with the FRP plate.Then the debonding strain is represented using the average strain of FRP plate within IDA,which decreases with an increase of FRP plate thickness.The experimental values of the debonding strain of SHCC beam reinforced with FRP plate are close to those predicted by the JSCE’s equation.

Fabrication and properties of binder for powder extrusion molding

By optimizing formulation and fabrication methods, a new binder forplasticizing powder extrusion molding of hard metal, with excellent integrated properties anduniform distribution characters, has been developed. Thermal debonding mechanism and the extrudingtheological behaviours have been studied. The technology of fabrication of binder and thermaldebonding process have also been investigated. Using the novel binder, the hard-metalextrusion-molding rods with diameter up to 25mm, have been manufactured.

Effect of Interfacial Strength on the Flexural Behavior of Glass Fiber Reinforced Polymer(GFRP) Reinforced Concrete Beam

The reinforcement/matrix interfacial strength has been considered as the key factor when glass fiber reinforced polymer(GFRP) bar is mixed with concrete. In this paper, based on micromechanics, fourpoint bending numerical models with and without glass fiber of different interfacial strength have been set up to simulate the damage process of GFRP reinforced concrete beam. The results show that the higher the interfacial strength is, the higher the ultimate bearing capacity of beams, and the less the opening width and height of cracks will be reached. Furthermore, mixing of glass fibers has less influence on the damage process when the interfacial strength is weak, however, it can help to improve the ultimate bearing capacity of the beams, retard the expansion of cracks and improve the toughness when the interfacial strength is high.